Partially Premixed Combustion Mode Research Articles

Performance of a Methanol-Fueled Direct-Injection Compression-Ignition Heavy-Duty Engine under Low-Temperature Combustion Conditions

Low-temperature combustion (LTC) concepts, such as homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC), aim to reduce in-cylinder temperatures in internal combustion engines, thereby lowering emissions of nitrogen oxides (NOx) and soot. These LTC concepts are particularly attractive for decarbonizing conventional diesel engines using renewable fuels such as methanol. This paper uses numerical simulations and a finite-rate chemistry model to investigate the combustion and emission processes in LTC engines operating with pure methanol. The aim is to gain a deeper understanding of the physical and chemical processes in the engine and to identify optimal engine operation in terms of efficiency and emissions. The simulations replicated the experimentally observed trends for CO, unburned hydrocarbons (UHCs), and NOx emissions, the required intake temperature to achieve consistent combustion phasing at different injection timings, and the distinctively different combustion heat release processes at various injection timings. It was found that the HCCI mode of engine operation required a higher intake temperature than PPC operation due to methanol’s low ignition temperature in fuel-richer mixtures. In the HCCI mode, the engine exhibited ultra-low NOx emissions but higher emissions of UHC and CO, along with lower combustion efficiency compared to the PPC mode. This was attributed to poor combustion efficiency in the near-wall regions and engine crevices. Low emissions and high combustion efficiency are achievable in PPC modes with a start of injection around a crank angle of 30° before the top dead center. The fundamental mechanism behind the engine performance is analyzed.

Comprehending flame development and misfire at advanced engine conditions: Detailed experimental characterizations and machine learning-assisted kinetic analyses

Through comprehensive experimental and modeling efforts, this work unravels the underlying mechanisms governing flame development and misfire at advanced engine conditions that are representative of low-load and lean blow-out operations. Toward this, preliminary heat release, autoignition, and flame developing patterns are characterized, via a case study of n-heptane, at ultra-lean conditions in a well-controlled optical engine under various combustion modes including homogeneous charge compression ignition (HCCI), partially premixed combustion (PPC), and reactivity-controlled compression ignition (RCCI). Changes in preliminary heat release and flame developing patterns at three overall equivalence ratios (0.12, 0.18, and 0.24) are first characterized under the PPC mode. Flame development characteristics including flame areas and number of initial flame kernels at close-to-misfire conditions are further extracted and compared across the HCCI, RCCI, and three PPC modes, with two distinctive and one transition regimes identified. Further analyses indicate that sustainable flame development and misfire are largely controlled by the spatial distribution of local equivalence ratio (phi) and local temperature in the mixture, which dictate the initial flame kernel generation and the subsequent flame propagation through localized preliminary heat release and autoignition. Chemical kinetic modeling is also undertaken, using a recently updated gasoline chemistry model, in conjunction with a backpropagation neural network, where the predicted ignition delay map well captures the different regions of flame development. Further kinetic analysis and heat rate of production per reaction analysis corroborate the CH2O planar laser-induced fluorescence experiments and highlight the important chemical kinetics that govern the initial flame development patterns.

Partially premixed combustion of diesel-di-ethyl ether blends in light-duty commercial engine

Diethyl ether (DEE) has evolved as a potential alternative for partial diesel replacement in compression ignition (CI) engines. In this study, a light-duty commercial diesel engine was operated in partially premixed combustion (PPC) mode. This work investigated the engine characteristics with lower and higher DEE-diesel blends in PPC mode. The split ratio and the start of the main injection (SoMI) timings were maintained at 70:30 (Main: Pilot) and 9° bTDC, respectively, while the start of the pilot injection (SoPI) timing was varied from 25 to 40° bTDC. The performance, combustion, and regulated emission characteristics of the PPC engine were compared with the conventional diesel combustion (CDC) mode as a baseline. The PPC mode exhibited higher/ comparable brake thermal efficiency (BTE) than the baseline CDC mode. The DEE-diesel blends exhibited higher BTE than the diesel in PPC mode generally. The DEE40 showed greater improvement in BTE than DEE20 at lower brake mean effective pressure (BMEP). PPC mode fuelled with diesel and diesel-DEE blends showed lower brake specific fuel consumption (BSFC) than baseline CDC mode at lower BMEP, but it was comparable at higher BMEP. PPC mode showed higher carbon monoxide (CO) emissions than baseline CDC mode. CO emissions increased with advancing SoPI timing for all BMEPs, except 5.6 bar. Although, the PPC mode exhibited higher hydrocarbon (HC) emissions than the CDC mode. Diesel-DEE blends showed slightly higher HC emissions than diesel in PPC mode at lower engine BMEPs. NOx emissions in the PPC mode were 1.5 to 2 times the baseline CDC mode. This indicated that the PPC mode engine performance requires further optimization similar to CDC mode, which had optimized exhaust gas recirculation (EGR), and significantly reduced the nitrogen oxide (NOx) emissions.

An attempt to implement partially premixed combustion strategy in a multi-cylinder CRDI engine: A detailed experimental and wavelet transform analysis

The main objective of this research was to effectively employ the n-butanol fuel in advanced combustion strategy ‘partially premixed combustion (PPC)’ and provide the maximum possible replacement of diesel fuel. An experimental study was conducted on a multi-cylinder turbocharged intercooler CRDI engine to optimize the injection timings and diesel/n-butanol blended fuels based on performance, emissions, and combustion characteristics, including cyclic variability, in order to achieve the PPC strategy without compromising thermal efficiency significantly. The combustion characteristics analysis indicate that adding n-butanol fuel into diesel fuel combined with advanced injection (16° CA BTDC) timing leads to a drastic decrease in smoke emission as well as increases in NOX emission than diesel fuel. However, DB20 fuel at 8° CA BTDC injection timing achieved low-temperature PPC mode and reduced NOX (8.08%↓) and smoke (48%↓) emissions simultaneously compared to diesel fuel at 2 bar BMEP engine load. In addition, wavelet power spectrum (WPS) and global wavelet spectrum (GWS) analysis were utilized to investigate the temporal and spectral fluctuations in the partially premixed combustion parameter (indicated mean effective pressure) at optimum injection timing (8° CA BTDC). Results reveal that DB20 possesses lesser cyclic variations with higher frequency (at period 2–6 cycles) intermittent fluctuation in partially premixed combustion at 2 bar BMEP engine load than other tested fuels. A statistical analysis of IMEP variations was also performed to validate the temporal variation in the IMEP time-series signals of the WPS results. The results of WPS could be used to establish an efficient engine control method based on in-cylinder pressure feedback.

Study of the Combustion Process of a Homogeneous Charge Compression Ignition (HCCI) Engine and a Partially Premixed Combustion (PPC) Mode of a Compression Ignition Engine Using Natural Gas as an Alternative Fuel

In order to investigate a viable approach to achieving high efficiencies and low nitrogen oxide (NOX) emissions, this paper presents the application of a homogeneous charge compression ignition (HCCI) engine and the partially premixed combustion (PPC) mode applied to a heavy diesel engine. The effect of carbon dioxide (CO2) fraction on combustion parameters was analyzed and discussed in detail. For this purpose, on the one hand, ANSYS CHEMKIN-Pro software was used to perform simulations of a closed homogeneous reactor under conditions relevant to HCCI engines, and on the other hand, ANSYS-Fluent software was used by adding a CO2 fraction varying from 20% to 58% to methane fuel to study 2D flow simulation by applying a PPC combustion mode to predict the distribution of various output parameters such as in-cylinder temperature, in-cylinder pressure and emissions. In comparison with the two presented models, it was found that the HCCI engine showed a lower NOX level than the PPC mode and this was due to the lower in-cylinder temperature in the HCCI engine.

Impact analysis of partially premixed combustion strategy on the emissions of a compression ignition engine fueled with higher octane number fuels: A review

This paper reviews the effect of a partially premixed combustion strategy on the exhaust emission of a diesel engine fuelled with high research octane number fuels. For the past few years, researchers have been aiming to decrease the soot and NOx emission simultaneously and maintain high engine efficiency in diesel engines by improving the combustion which is achieved by optimizing the fuel injection tactics. Single fuel and split fuel (pilot/main) injection tactics are used in the partially premixed combustion (PPC) strategy of the diesel engine to achieve stable combustion and less soot and NOx emission. The PPC strategy in diesel engine can be easily achieved with high octane number fuel resulting in prolonged ignition delay. The air/fuel mixing would be improved by virtue of longer ignition delay (ID) and as a result, soot emission decreases in the partially premixed compression ignition (PPCI) engine. Additionally, higher octane number fuels (i.e., Ethanol, Methanol and Butanol) improve the charge cooling effect, resulting in reduced NOX emissions. Thus, the combined use of PPC mode diesel engine along with high octane number fuels in modern CI engine could be a favourable upcoming solution to meet the strict exhaust emission legislation and reduce the dependence on fossil fuels.

Auto-ignition characteristics of high-reactivity gasoline fuel using a gasoline multi-hole injector

Gasoline compression ignition (GCI) engines have proven to be a highly efficient engine technology with reduced emissions. The high efficiency and reduced emissions of GCI engines heavily rely on the stratification of the fuel being injected directly into the cylinder through multi-hole injectors. Therefore, it is critical to understand the fuel stratification and auto-ignition behaviors of the fuels used in GCI engines. Thus, in this work, the auto-ignition characteristics of high-reactivity and low carbon gasoline fuel (RON 77) were studied in an optically accessible constant volume chamber. A customized high-pressure gasoline multi-hole injector was used. Reactive tests were performed at two different ambient pressures (20 and 30 bar), three different ambient temperatures (800, 900, and 1000 K), three different oxygen concentrations (10, 15 and 21%) and three different injection pressures (100, 300 and 450 bar). The auto-ignition of fuel was achieved with varying ignition delay based on the experimental conditions tested. It was found that the operating conditions profoundly influences the diffusion and partially-premixed combustion mode. For high ambient pressures, temperatures, oxygen concentrations, injection pressure, and combinations, diffusion combustion mode was observed, and partially premixed combustion mode was observed at lower ambient pressures, temperatures, oxygen concentrations, injection pressures, and their combinations.

Influence of Injection Strategies on Engine Efficiency for a Methanol PPC Engine

<div class="section abstract"><div class="htmlview paragraph">Partially premixed combustion (PPC) is one of several advanced combustion concepts for the conventional diesel engine. PPC uses a separation between end of fuel injection and start of combustion, also called ignition dwell, to increase the mixing of fuel and oxidizer. This has been shown to be beneficial for simultaneously reducing harmful emissions and fuel consumption. The ignition dwell can be increased by means of exhaust gas recirculation or lower intake temperature. However, the most effective means is to use a fuel with high research octane number (RON). Methanol has a RON of 109 and a recent study found that methanol can be used effectively in PPC mode, with multiple injections, to yield high brake efficiency. However, the early start of injection (SOI) timings in this study were noted as a potential issue due to increased combustion sensitivity. Therefore, the present study attempts to quantify the changes in engine performance for different injection strategies. Simulations were performed on a heavy-duty multi-cylinder compression ignition engine fueled with methanol. Two operating conditions with different engine load were chosen from the European stationary cycle. Three different injection strategies were applied: 1) SOI > −160°ca aTDC 2) SOI > −40°ca aTDC 3) SOI > −25°ca aTDC. The engine settings were selected to maximize the brake efficiency for each case and the sensitivity of combustion to inlet conditions was analyzed. For the high load operating point, the brake efficiency was 2.2 %pt. higher for case 1 compared to case 3, while this difference was only 0.5 %pt. for the low load operating point. However, the combustion phasing for case 1 and 2 at the high load point proved to be very sensitive to inlet temperature, inlet pressure and oxygen concentration.</div></div>

Combustion stability study of partially premixed combustion with low-octane fuel at low engine load conditions

The study aims to investigate the sensitivity of combustion stability to the intake air temperature for partially premixed combustion (PPC). The experiments were carried out in a full view optical engine at low load condition. The ω shape optical piston crown as same as the actual product piston, rather than the flat crown piston used in the previous study, was employed for the present experimental test. The continuous-fire mode rather than the skip-fire mode was used to run the optical engine ensuring the similarity to the actual engine operating conditions. The interaction among fuel spray jets, piston and cylinder wall was visualized by fuel-tracer planar laser-induced fluorescence. The high-speed combustion images were processed to determine the combustion stratification based on the natural flame luminosity. The combustion phasing, maximum in-cylinder pressure, and indicated mean effective pressure (IMEP) were compared at various intake temperatures. The results showed that the lower intake temperature could be used for achieving better combustion stability at low load condition along with the retarded CA50, the lower maximum in-cylinder pressure, and the higher IMEP. 70 °C was the lower limit of intake temperature to achieve stable PPC operation with the single-injection strategy. The same trend of the combustion characteristics with respect to the start of injection timing was confirmed at various intake temperatures. The combustion stratification analysis indicated more inhomogeneous low-temperature combustion with decreased natural flame luminosity and increased soot emission when the intake temperature reduced from 120 °C to 70 °C. Nitrogen oxides emission decreased when compared to the higher intake temperature cases at the expense of increased unburned hydrocarbon and carbon monoxide emissions at PPC mode. The fuel tracer measurements showed that most of the injected fuel hit on the piston top and only less amount of fuel was injected into the piston crown bowl at PPC mode due to the wider spray umbrella angle. The fuel trapped in crevice zone was verified as an important source for the unburned hydrocarbon and carbon monoxide emissions at PPC mode. The injector dribbling during the late stage of combustion attributed to soot formation. The injector with a relatively narrow spray umbrella angle was suggested for optimized interaction among the fuel spray jets, piston and the cylinder wall at PPC mode.

Simultaneous 36 kHz PLIF/chemiluminescence imaging of fuel, CH2O and combustion in a PPC engine

The requirements on high efficiency and low emissions of internal combustion engines (ICEs) raise the research focus on advanced combustion concepts, e.g., premixed-charge compression ignition (PCCI), partially premixed compression ignition (PPCI), reactivity controlled compression ignition (RCCI), partially premixed combustion (PPC), gasoline compression ignition (GCI) etc. In the present study, an optically accessible engine is operated in PPC mode, featuring compression ignition of a diluted, stratified charge of gasoline-like fuel injected directly into the cylinder. A high-speed, high-power burst-mode laser system in combination with a high-speed CMOS camera is employed for diagnostics of the autoignition process which is critical for the combustion phasing and efficiency of the engine. To the authors’ best knowledge, this work demonstrates for the first time the application of the burst-system for simultaneous fuel tracer planar laser induced fluorescence (PLIF) and chemiluminescence imaging in an optical engine, at 36 kHz repetition rate. In addition, high-speed formaldehyde PLIF and chemiluminescence imaging are employed for investigation of autoignition events with a high temporal resolution (5 frames/CAD). The development of autoignition together with fuel or CH2O distribution are simultaneously visualized using a large number of consecutive images. Prior to the onset of combustion the majority of both fuel and CH2O are located in the recirculation zone, where the first autoignition also occurs. The ability to record, in excess of 100 PLIF images, in a single cycle brings unique possibilities to follow the in-cylinder processes without the averaging effects caused by cycle-to-cycle variations.

Homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC) in compression ignition engine with low octane gasoline

The present study investigated the in-cylinder combustion for low octane 70 primary reference fuel (PRF70) by the method of the flame index during the transition from homogeneous charge compression ignition (HCCI) combustion to partially premixed combustion (PPC). Full cycle engine simulations were performed using CONVERGE™, coupled with chemical kinetics. Good agreements between the simulations and experiments were achieved at HCCI and PPC combustion modes. The fully premixed HCCI mode was achieved at the earliest injection timing of −180 CAD aTDC with the combustion temperature below 1600 K, where the formation of soot and NOx can be successfully avoided. For the injection timing of −100 CAD aTDC, the premixed charge compression ignition (PCCI) was achieved where the premixed combustion clouds were mainly distributed in the piston top-land zone and were surrounded by the diffused combustion that occurs in the piston bowl and the periphery of piston top. Less premixed flames were formed in piston top and surrounded by more diffusion mixtures at PPC mode. The in-cylinder HO2 evolution profile displayed two bumps which were distributed in low temperature zone and high temperature zone respectively. The spatial and temporal evolution of HO2 is very similar to the distribution of premixed flames.

Experimental investigation of injection strategies on particle emission characteristics of Partially-premixed low temperature combustion mode

Compared with the traditional combustion mode, Partially-premixed combustion (PPC) mode can reduce the particulate emissions. However, the particulate emission characteristics of this strategy do not have a clear conclusion. Based on a 4-cylinder turbocharged diesel engine, this paper aims at studying the influence of the fuel injection timing, injection pressure and the pilot injection to the engine combustion and emission characteristics under the condition of single-injection and split-injection PPC mode respectively. The results show that the early-injection PPC under single-injection strategy can reduce the number, mass and GMD of particles obviously. Increasing the injection pressure reduces the number concentration of particulates of the accumulation mode, but increases the number concentration of the nucleation mode. The total number and mass concentration reach up to the peak with the pilot injection timing is −25 °CA ATDC. The concentration decreases slightly with the advance of pilot injection timing. With the application of the pilot injection strategy, the peak values of the number concentration and mass concentration occur in the accumulation mode region. The NOx emission is higher in the early-injection PPC mode under the single-injection strategy. By contrast, the split strategy can control PM and NOx emissions simultaneously.

GTLine – Gasoline as a potential CN suppressant for GTL

The main driver to investigate low temperature combustion concepts, such as partially premixed combustion (PPC), is the promise of low particulate matter (PM) and nitric oxide (NOx) emissions. A critical prerequisite for PPC is to temporally isolate the fuel injection and combustion events. In practice, exhaust gas recirculation (EGR) is applied in order to sufficiently extend the ignition delay to that effect. Hereby, in general, higher EGR rates are necessary for fuels with higher cetane numbers (CN).Against this background, the objective of this paper is to investigate the efficacy, with respect to PM-NOx emissions and engine efficiency, of gasoline as a potential gas-to-liquid (GTL) CN suppressant in various dosages. The performance of the resulting GTLine blend will be evaluated under PPC operating conditions in a heavy-duty direct-injected diesel engine.Setting aside for a moment any potential practical issues (e.g., flash point, vapor pressure) that fall outside the scope of this study, our data suggest that blending gasoline to otherwise high CN GTL appears to be a promising route to improve not only the efficiency, but also PM and NOx emissions, particularly when operating in PPC mode. Interestingly, this benefit is notwithstanding the high aromaticity of the gasoline compared to GTL.Given the ongoing dieselization trend and associated surplus of gasoline in many regions, notably Europe, along with the fact that the cost price of gasoline is significantly lower than that of GTL, the proposed GTLine approach promises to be a cost effective way to accommodate GTL in a world wherein low temperature combustion concepts, such as PPC, appear to be really taking off.

Naphtha vs. dieseline – The effect of fuel properties on combustion homogeneity in transition from CI combustion towards HCCI

The scope of this research study pertains to compare the combustion and emission behavior between naphtha and dieseline at different combustion modes. In this study, US dieseline (50% US diesel + 50% RON 91 gasoline) and EU dieseline (45% EU diesel + 55% RON 97 gasoline) with derived cetane number (DCN) of 36 are selected for experimentation in an optical engine. Besides naphtha and dieseline, PRF60 is also tested as a surrogate fuel for naphtha. For the reported fuel with same RON = 60, the effect of physical properties on combustion homogeneity when moving from homogenized charge compression ignition (HCCI) to compression ignition (CI) combustion is studied.The combustion phasing of naphtha at an intake air temperature of 95 °C is taken as the baseline data. The engine experimental results show that higher and lower intake air temperature is required for dieseline mixtures to have same combustion phasing as that of naphtha at HCCI and CI conditions due to the difference in the physical properties. Especially at HCCI mode, due to wider distillation range of dieseline, the evaporation of the fuel is affected so that the gas phase mixture becomes too lean to auto-ignite. However, at partially premixed combustion (PPC) conditions, all test fuels required almost same intake air temperature to match up with the combustion phasing of baseline naphtha. From the rate of heat release and combustion images, it was found that naphtha and PRF60 showed improved premixed combustion when compared dieseline mixtures. The stratification analysis shows that combustion is more stratified for dieseline whereas it is premixed for naphtha and PRF60. The level of stratification linked with soot emission showed that soot concentration is higher at stratified CI combustion whereas near zero soot emissions were noted at PPC mode.

Exhaust PM Emissions Analysis of Alcohol Fueled Heavy-Duty Engine Utilizing PPC

The focus has recently been directed towards the engine out soot from Diesel engines. Running an engine in PPC (Partially Premixed Combustion) mode has a proven tendency of reducing these emissions significantly. In addition to combustion strategy, several studies have suggested that using alcohol fuels aid in reducing soot emissions to ultra-low levels. This study analyzes and compares the characteristics of PM emissions from naphtha gasoline PPC, ethanol PPC, methanol PPC and methanol diffusion combustion in terms of soot mass concentration, number concentration and particle size distribution in a single cylinder Scania D13 engine, while varying the intake O2. Intake temperature and injection pressure sweeps were also conducted. The fuels emitting the highest mass concentration of particles (Micro Soot Sensor) were gasoline and methanol followed by ethanol. The two alcohols tested emitted nucleation mode particles only, whereas gasoline emitted accumulation mode particles as well. Regarding soot mass concentration measurements; methanol never exceeded 1.6 mg/m3 while when operating on gasoline this value never descended below 1.6 mg/m3. From this result it can be concluded that the main contributor to PM mass emissions is mainly increasing CMD (Count Mean Diameter) in the accumulation mode size range, but can in diffusion combustion also be caused by a high amount of nucleation mode particles. A probable cause of higher particle number emissions, when running the engine on methanol compared to ethanol, is the corrosiveness of the fuel itself. Except for the ultra-low PM mass emitted from alcohol combustion, it is also possible to alter the EGR concentration with a higher level of freedom without having to consider the NOX - soot tradeoff.

Comparison of Negative Valve Overlap (NVO) and Rebreathing Valve Strategies on a Gasoline PPC Engine at Low Load and Idle Operating Conditions

Gasoline partially premixed combustion (PPC) has the potential of high efficiency and simultaneous low soot and NOx emissions. Running the engine in PPC mode with high octane number fuels has the advantage of a longer premix period of fuel and air which reduces soot emissions. The problem is the ignitability at low load and idle operating conditions. In a previous study it was shown that it is possible to use NVO to improve combustion stability and combustion efficiency at operating conditions where available boosted air is assumed to be limited. NVO has the disadvantage of low net indicated efficiency due to heat losses from recompressions of the hot residual gases. An alternative to NVO is the rebreathing valve strategy where the exhaust valves are reopened during the intake stroke. The net indicated efficiency is expected to be higher with the rebreathing strategy but the question is if similar improvements in combustion stability can be achieved with rebreathing as with NVO. The results show that the rebreathing valve strategy has similar improvements on combustion stability as NVO when the same fuel injection strategy is used. This work also includes results with the NVO valve strategy where a fuel injection is added during the NVO. When a fuel injection is added during the NVO, an additional improvement on combustion stability can be seen which is unmatched by the rebreathing valve strategy. (Less)